1. Field of the Invention
The present invention relates generally to the field of integrated circuit fabrication, and more specifically to a structure for static random access memory cells with tunnel diodes and a method for making the same.
2. Description of the Related Art
The traditional SRAM cell consists of six transistors configured as cross-coupled inverters to form a flip-flop. The minimum attainable cell size has remained at approximately 120 F.sup.2 (where F denotes the feature size--the minimum line width and/or minimum space between lines) using standard planar technology. The drive to achieve further reductions in cell size has led to the use of vertical transistors, but even with this technology, which is more complicated and hence more costly, the feature size cannot be reduced to below 32 F.sup.2. Thus, although absolute SRAM cell size can be improved with reductions in feature size corresponding to advances in lithography technology, further reductions in SRAM cell size require changes in cell configuration. As used herein, cell configuration refers to the components (e.g. transistors, diodes) of the cell and their interconnection. Configuration has a different meaning from structure. Structure, as used herein, refers to the physical topography of the fabricated cell.
Several different SRAM cell configurations have been proposed. Some of these new structures exploit latchup as a mechanism of cell operation. Other new SRAM cell configurations make use of bipolar base current reversal. Examples of such configurations can be found in Koji Sakui et al., A New Static Memory Cell Based on Reverse Base Current (RBC) Effect of Bipolar Transistors, 1988 IEDM Digest of Technical Papers, pp. 44-47, and in U.S. Pat. No. 5,594,683. These and other new SRAM cell configurations do achieve smaller cell size.
Each of these alternative configurations, however, suffers from an important drawback--high standby power consumption. Standby power consumption is the amount of power used by a cell when neither read nor write accesses are occurring. This drawback is especially problematic in situations such as BBRAM (battery backed-up RAM) where low standby power consumption is crucial.
Another alternative configuration is disclosed in van der Wagt et al., RTD/HFET Low Standby Power SRAM Gain Cell, IEEE Electron Device Letters, vol. 19, No. 1 (January, 1988). This configuration uses only two tunnel diodes and a single FET, but still suffers from relatively high standby power consumption (approximately 50 nanowatts per cell). The high standby power consumption is partially due to the fact that the cell described in van der Wagt is fabricated using III-V technology (integrated circuits fabricated on substrates such as GaAs comprising combinations of elements from groups III and V of the periodic table).
A circuit diagram of this SRAM cell 10 is shown in FIG. 1. Two tunnel diodes 14, 16 are connected in series between a voltage source 12 and ground such that the diodes 14, 16 are both forward biased. The storage node 15 between the diodes 14, 16 is connected to the drain 18a of a field effect transistor 18. The source 18b of transistor 18 is connected to the bit line 20, while the gate 18c of transistor 18 is connected to the word line 22. In this configuration, the transistor 18 allows access to the storage node 15 much as a transistor controls access to a storage capacitor in a conventional DRAM cell.
FIG. 2 is a plot of the current vs. voltage characteristic curve 40 of the tunnel diodes 14, 16. The vertical axis 42 is in milliamps, while the horizontal axis 44 is in volts. The curve 40 exhibits a relative minimum, or valley, current at approximately 0.3 volts (point A in FIG. 2). As can be seen from line 46, this same current also occurs at a forward bias of approximately 0.02 volts (point B in FIG. 2). Thus, a combination of two diodes 14, 16 forward biased in series with a total bias of 0.32 volts will have a current equal to the valley current indicated by line 46, with 0.3 volts across one diode and 0.02 volts across the other. Since either diode can have either voltage, two stable states for the diode 14, 16 combination exist. The node 15 thus acts as the storage node, which can remain stable at either 0.3 or 0.02 volts. The stability of the cell states is determined by the value of the voltage across the diode pair 14, 16 as illustrated in FIG. 3. The node 15 can be set to either of these states by applying the desired voltage to the bit line 20 and raising the word line 22 voltage to turn on the access transistor 18. Reading may be accomplished as in a DRAM cell by using voltage sense amplifier to sense the voltage on the bit line 20 after raising the word line 22 voltage to connect the node 15 to the bit line 20. Because the node 15 is in a self-sustaining stable voltage state, current sensing may also be used to read the cell state.
What is needed is a compact cell structure and corresponding fabrication method that realizes the above-discussed SRAM circuit configuration in a small amount of space while improving standby mode power consumption.